Abstract

Temperature-dependent sex determination was studied in a northern population of painted turtles (Chrysemys picta) in both laboratory and field. Eggs incubated at constant temperatures of 30 and 32 °C produced females only, whereas those kept at 22, 24, and 26 °C produced males only. Both sexes occurred at 20 and 28 °C. The threshold temperatures (temperatures producing 50% males) were estimated to be 27.5 and 20.0 °C, and were similar to those reported for more southerly populations of C. picta. In both 1983 (a relatively warm summer) and 1984 (an average summer), temperatures in natural nests regularly fluctuated above and below both threshold temperatures. Mean nest temperatures were warmer in 1983 than in 1984, but were not useful to predict nest sex ratios. Mean nest temperatures were not similar to constant temperatures in their effect on sex ratio. Sex ratios in nests could be described best by the total number of hours for which the temperature at each nest was intermediate to the two threshold temperatures. Sex ratios (proportion male) of hatchlings in 1983 and 1984 were similar and female biased (0.12 and 0.13, respectively).